1. Field of the Invention
The present invention relates to a method and apparatus for recording and reproducing information on a plural-track record disk. The invention is particularly directed to a technique for increasing the capacity of a disk to store data by enabling the data to be recorded along a plurality of closely-spaced, concentric circular tracks on the record disk.
2. Description of the Prior Art
Plural-track magnetic disks of the "flying-head" type are commonly used for recording retreivable informational data in computers and other data processing systems. A factor limiting the track density, and therefore the disk capacity, is the precision with which the head can be positioned with respect to a selected track. This is particularly true when the disk is recorded on one unit and played-back on another unit, or when parts of a disk are recorded or played-back on different units. Positional inaccuracies arise from "drift" or other cumulative causes, such as wear and distortion with time and use of mechanical parts; thermal effects both after switch-on and during use, such as those arising because of temperature coefficient variations relative to the frame and disk; humdity variations, particularly in the case of flexible disks; and spindle eccentricity of one unit relative to another unit.
The "open loop" technique for positioning the head with respect to a selected track, in which the head is moved a measured distance from a reference point on the frame, does not compensate for the above positional inaccuracies. Systems have been proposed including specially-written tracks, or "dedicated tracks", which convey information concerning magnitude and direction of the displacement of the reading head from the track center or axis. Such systems, however, require that a substantial recording surface be allocated to the specially-written or dedicated tracks for purposes of conveying the positional information, and therefore they substantially reduce the surface of the record disk available for recording the retreivable informational data. In addition, such dedicated tracks convey directly their own positions, and only indirectly the position of the data tracks, and therefore do not completely compensate for possible variations between the two arising from the above-discussed positional inaccuracies.